The present invention relates to passive shimming of magnetic resonance magnets to improve magnetic field homogeneity in the bore of the magnet and more particularly, passive shims for creating desired combinations of (3,2) and (3-2) harmonic terms.
Obtaining good quality magnetic resonance images requires a main static magnetic field with high field homogeneity throughout the imaging volume of the magnet. The field homogeneity varies from magnet to magnet because of deviations of the magnetic windings from a specified shape and position due to manufacturing variations. In addition, the presence of ferromagnetic material (such as building structural supports) in the vicinity of the magnet can degrade image quality obtained during a scan.
The magnetic field in the bore of a magnet can be represented as a series of spherical harmonics expanded about the origin of the magnet coordinate system with one of the harmonic terms having no spatial variation and representing the desired constant field. The origin of the coordinate system is situated at a central point in the magnet bore. The spherical harmonics are represented as (n,m) where n is the degree and m is the order of the spherical harmonic space. The elimination of harmonics other than the harmonic representing the constant uniform field would increase the overall homogeneity of the field.
Correction coils are typically used to improve the magnetic field homogeneity. The correction coils are specially shaped windings situated in the bore of the magnet, capable of creating different field shapes associated with the unwanted harmonics. The field shapes created by the correction coils can be superimposed on the inhomogeneous main field to perturb the main field in a manner to increase the overall field uniformity. Each correction coil typically has its own power supply to provide the correct current flow to the coil windings to produce the desired amount of the compensating field shape to cancel the existing field shape in the magnet.
Some early magnets, which are still being used, do not have correction coils for the (3,2) and (3,-2) terms in the spherical harmonic expansion of the magnetic field. A design to eliminate these harmonics using passive shims has been proposed by the Oxford Magnet Technology Co. (OMT) and is shown in FIG. 1. This design, however, introduces a high level of high order harmonic contaminants, much more than the design of the present invention. High order field terms cannot usually be removed by the correction coils of the magnets since the number of correction coils are limited in number and only correct lower order harmonics. If the (3,2) and (3,-2) harmonic terms can be eliminated without introducing higher order harmonics, which cannot be compensated for, the field homogeneity and therefore the image quality can be improved.
It is an object of the present invention to provide a simple and reliable means for creating an arbitrary combination of (3,2) and (3,-2) harmonic terms without the creation of higher order harmonic contaminants.